The desired strength and durability of concrete can’t
be achieved unless compaction of concrete has been done well
using good and suitable vibrators, writes JAGVIR GOYAL.
It’s a simple fact. However good the quality of concrete
ingredients may be, however strict the control on its water-
cement ratio, however sophisticated the equipment for the
mixing may be, transportation and placement of concrete, desired
strength and durability of concrete can’t be achieved
unless the compaction of concrete has been done well and in
the right manner. Despite the aggregates complying with the
requirements of IS 383, the water being of potable quality
and cement being fresh and strong, a dense mass of concrete
is obtained only after full expulsion of air from it through
compaction methods. Compaction of concrete should, therefore,
never be taken lightly.
An increased use of ready mix concrete all over the country
is being seen a lot, these days. Transit mixers carrying concrete
can now be seen even in small towns and cities. Here also,
smaller lots are supplied on only a dumping basis and the
purchasers have to make arrangements for the placement and
compaction of concrete. Larger batches are placed directly
by the RMC supplier. In both cases, however, for best results
and serviceability, proper arrangements for compaction of
concrete are required.
While talking of equipment for concrete compaction, only one
word comes to mind: the vibrator. Vibrators are the sole pieces
of successful equipment for compaction of concrete. There
were times when no mechanical machinery was available and
rods and tamps were used for hand compaction of concrete.
Today, we can’t even imagine that such rough methods
were ever used. Mechanical compaction is mandatory. However,
mere arrangement of vibrators is not enough. The choice of
the right vibrator is equally important. Proper compaction
of concrete is possible only if good and suitable vibrators
are available for it.
Types of vibrators
The vibrators for concrete compaction are divided into two
classes: internal vibrators and external vibrators.
These are also known as needle vibrators, immersion vibrators
or poker vibrators. These produce vibrations directly in the
concrete. The engine used to run them can be electric or pneumatically
driven, petrol or diesel driven. The needles are of varying
diameters between 25 mm to 155 mm. the main sizes are 25 mm,
40 mm, 50 mm, 60 mm, 75 mm, 100 mm, 110 mm, 140 mm and 155
mm. 40 mm and 60 mm are com¬monly used sizes. Petrol and
diesel driven vibrators prove useful when electricity or compressed
air sources are not available. These vibrators can easily
be lifted by the operator and shifted to convenient locations
for immersing the needle into a concrete mass.
Internal vibrators are suitable for jobs involving mass concreting
such as raft foundations, pile caps or jobs where a good depth
of concrete is involved. These vibrators are more common than
external vibrators because of the ease in their use. As a
single person can shift them from place to place, in smaller
jobs like housing, roof slabs, beams, columns etc, only internal
vibrators are put to use.
Internal vibrators can produce 10,000 to 20,000 vibrations
per minute at amplitudes of 0.8 mm to 1.6 mm and ensures fast
compaction of concrete. These vibrators consist of a motor
unit and a flexible hose which is attached to the needle.
The hose encloses a shaft in it which is driven by the engine.
The shaft produces vibrations in the needle which transfers
the same to the concrete on its immersion into the concrete
mass. Vibrations are caused by eccentric weights attached
to the shaft or the motor or the rotor of the vibrating part.
The movement of these eccentrically placed weights causes
vibrations in the poker which on transmission to concrete,
results in the expulsion of entrapped air and compaction of
concrete. Internal vibrators can be both of motor-in-head
type as well as separate drive unit type. The length of flexible
hoses varies from 1 m to 6 m but may be even longer. Flexible
hoses are steel wire braided and able to resist wear. The
tube length is kept as 300 mm to 500 mm. Motors are generally
of 2 to 4 HP. Weight of vibrators varies from as low as 6
kg to even 50 kg. The shaft alone may weigh more than the
drive unit. The frequency of pokers for the pendulum type
vibrators is around 12,000 to 14,000 rpm while for the pneumatic
type, it ranges from 15,000 to 21,000 rpm.
To make the vibrators user-friendly, the drive units are
made doubly insulated and the flexible shafts are attachable
by pressing a button instead of screwing them up.
External vibrators are also known as surface vibrators or
form vibrators. Unlike internal vibrators, these don’t
come in direct contact with concrete but compact it by transferring
the vibrations through the forms or surfaces holding the concrete.
Needless to mention, the forms or surfaces holding concrete
in this case need to be strong enough to take the vibrations
caused by external vibrations.
External vibrators are best suited for pre-cast concrete
components like pre-stressed poles and other such products.
Sometimes, a mesh like reinforcement is used in concrete.
In such cases, external vibrators are better as it is not
possible for the poker to penetrate the mesh during compaction
of concrete. Certain constr¬uctions demand laying of heavy
reinforce¬ments at very little bar-to-bar spacing. In
such cases, form vibrators can be used. Concrete in road pavements
and canal linings is also better compacted through form vibrators.
The time of application of form vibrators is much more than
that for needle vibrators. The form work has also to be very
strong to avoid any sway or failure due to heavy vibrations
produced by form vibrators. Surface vibrators lose effectiveness
along the edges of floors as vibrations are not fully transmitted
due to area restriction. Surface vibrators aren’t used
much in India.
External vibrators have lesser frequency than internal vibrators,
the range being between 1,500 to 6,000 vibrations per minute.
The amplitude is around 0.4 mm to 0.5 mm. Therefore, the vibrations
produced by these vibrators reach 8 to 10 inch depths of concrete.
Their base pan is located on the forms for concrete. An electric
or pneumatic engine produces vibrations in the pan which transmits
them to the forms and in turn, forms transmit them to the
concrete. For surface vibrators, 6,000 rpm can be counted
as high speed, 3,000 rpm as medium and 1,500 rpm low speed.
The speed of pneumatic vibrators is , however, plus 9,000
rpm and can be as high as 20,000 rpm.
Preparing the plan
While preparing the plan for comp¬action of concrete,
the following points should be kept in mind:
• The vibrators must be of sufficient capacity to ensure
full compaction of concrete matching with the speed of its
placement. If vibrators have less capacity or are insufficient
in number, loose concrete will keep accumulating at site and
a situation will arise when some layers of it will get covered
by subsequent layers without compaction.
• The vibrators must be of suitable type for the job
in hand. Here, the spacing of reinforcement is the major deciding
factor. In some situations like turbo-generator deck or other
heavily loaded structures, spacing of steel is very small
and many layers of large diameter steel bars are to be laid.
In such situations it has to be seen that the vibrations reach
every corner of the formwork and the concrete gets fully compacted.
• Accessibility of all areas to be concreted is another
important factor. The vibrators chosen should be able to access
all parts. Often, long columns and high rise structures allow
only limited access. In such cases, the type of vibrators
to be used should be decided carefully. Inaccessible areas
may demand compaction of concrete through surface vibrators.
Deploying skilled operators
Needle vibrators need dynamic operators who are swift in moving
vibrator needles from one point to another at split second
reaction time. However, as the operators are required to handle
and move the heavy needles, they need to work in pairs. While
one operator operates the needle, the other should be shifting
the motor unit. After a suitable gap of say, one hour, they
should swap places. Whenever this arrangement is not followed,
the quality of compaction of concrete may suffer.
New trends in vibrators
There have been some good innov¬ations made by the vibrator
producing industry. A few of them are:
High cycle vibrators
While the common poker vibrators operate at a frequency of
50/60 Hz, high cycle vibrators operating at a frequency of
180 Hz are now available. The electric motors used in these
vibrators are three phase low induction motors instead of
single phase and are located within the vibrator head itself.
These vibrators are good for low slump concrete used in heavy
constructions. High frequency maximises their productivity.
These produce around 11,000 vibrations per minute and weigh
between 15 to 25 kgs.
Computer controlled vibrators
Computer controlled vibrators have a computerised arrangement
to supply more power to the head as soon as it meets resistance
on being placing in the concrete. This helps in maintaining
the motor rpm and frequency which otherwise gets reduced on
meeting resistance. These vibrators can operate even under
zero slump conditions.
Now, internal vibrator units with built- in electronic frequency
converters have been invented by the manufacturers. These
can be plugged to any wall socket and are easy to operate.
While using them, it has to be seen that total amperes of
the poker vibrator does not exceed the stated amperes of frequency
Sometimes the reinforcement is so dense that it is not possible
to insert needles in between the bars. For such cases, a special
blade attachment has been invented and this blade replaces
the needle. It can now be inserted between the bars and compaction
can be done.
The vibrator industry
Many small and big industrial units are producing concrete
vibrators in India. Dynapac, Aquarius, Global Impex, Univ¬ersal,
MK, Taurus are some of them. Dynapac has researched the subject
well and has produced both needle type as well as external
vibrators suitable for all small and big jobs. It has produced
pendulum vibrators, high frequency pokers powered by single
phase motors for small jobs, motor in head pokers and pneumatic
vibrators. It also provides technical support to calculate
the vibrator force required, numbers of vibrators required
and their positioning in case of external vibrators. Experience
has made the manufacturers knowledgeable on correct frequency
and amplitude choice and a variety of vibrators are now available.
Multiquip and Denver are producing high cycle vibrators these
days. In India, distributors like Sona Sales supply high frequency
concrete vibrators with elec¬tronic integrated invertors
manufactured by Vibromaster, Korea.
* Jagvir Goyal is the author of several technical books, a
columnist for technical journals, Deputy Chief Engineer Civil
in Punjab and the recipient of the TIET Distinguished Alumni
TO ACHIEVE BEST COMPACTION
• Always keep the needle vertical. Never lay it on the
concrete. Never force it into the concrete. Let it travel
to the bottom on its own.
• Don’t move the concrete laterally with the help
of the needle.
• Never over-vibrate. Over vibration will draw a laitance
layer on the surface of concrete by pulling out the cement
particles. Decide the time of vibration at a particular point
as per site experience. It may be somewhere between 20 to
60 seconds. Keep an eye on the rising bubbles of air. The
moment these bubbles stop appearing, compaction has been done
at that point. The lower the slump, the higher is the compaction
time at a point.
• Never pull out the needle from the concrete in an
abrupt manner. Such an action will leave a vertical hole in
the concrete. Withdraw it slowly.
• Don’t stop the vibrator with the needle immersed
in the concrete. If it is done, withdrawal of needle will
• Never run the vibrator in the air. Switch it on when
the needle has been vertically rested on the concrete surface.
• To avoid a cold joint, see that the needle penetrates
the current layer and enters the fully compacted layer below
it by three to four inches.
• Never rest the motor unit of the vibrators on the
steel reinforcement laid for concreting.
• Never allow the needle to touch the formwork or steel
reinforcement. Prolonged touching may cause travel of vibrations
WHILE USING INTERNAL VIBRATORS
• Check the spacing of reinforcement and know the minimum
spacing of the bars. Such should be the size of the needles
that these may easily pass through the spacing of steel bars.
• The diameter of the needle should be as large as possible.
It will allow better compaction of concrete.
• Assess the distance of the platform where the motor
unit of the vibrator will be kept from the pouring points.
Choose the length of flexible hose accordingly. Unnecessarily
long hoses when not required hinder work. Therefore, choosing
longest hoses is not always right.
• The more the workability or slump of concrete, the
less can be the diameter of the needle.
• Electrically driven internal vibrators prove cheaper
than petrol or diesel driven vibrators. Try to use electrically
• Pokers or needles often demand repair. The number
of damaged needles may be high if the right diameter of needles
has not been chosen or operators are not skilled in using
them in the right manner. Keep a good number of spare needles
available at site for quick replacement where required.
• Choose high frequency vibrators produced by reputed
manufacturers. See that the poker tubes and caps are made
of specially hardened magnesium steel alloy.
• Look for steel reinforced hoses.
• Look for a good guarantee period as vibrators are
liable to breakdown. Look for such vibrator models that require
least maintenance and whose spare parts are easily available.
• Whenever pneumatic vibrators are used, on completion
of work, the compressor should be first switched off and then